Machines and other vibrating structures generate characteristic vibrations during their normal operations. These vibrations can vary from a simple sinusoidal type, to a complex, multi-frequency, non-linear oscillation or random type. Changes in the oscillation form of vibrations emanating from such machinery can result from a simple change in operation, but they may also be the result of wear, failure or fatigue of mechanical parts. By analyzing and isolating the characteristics of the vibrations of a machine, early warning signs of machine failures can be detected. This information may be used to prevent unscheduled downtime of the machine, thereby increasing machine efficiency.
Vibrations are typically measured using analog vibration sensing elements, such as analog accelerometers, positioned on machinery at strategic locations. The vibration sensing elements are selected so that the observed amplitude and frequency range optimally measure the vibrations at a specific location. Separate analog filters, such as band-pass and low-pass filters, may be attached to the analog output of a vibration sensing element to further adjust the vibration signal measured by the vibration sensing element. The result is that a cornucopia of vibration sensing elements and filters may be distributed on a single machine.
The output of a vibration sensing element may also be subject to further analysis. Typical calculations can include the Root-Mean-Square Average (RMS), peak-to-peak amplitude, as well as a Fast Fourier Transform of the vibration signal, to determine the dominant frequencies in the signal. In simple vibration sensors containing a vibration sensing element, these calculations are typically accomplished in embedded hardware.
Where more complex analysis is desired, vibration signals are often digitized. The analog signals output from vibration sensing elements spread across a machine may be wired to a single point for analog-to-digital conversion. From these digital signals, not only may the above-mentioned typical calculations be performed, but also more sophisticated calculations such as modal analysis, correlations, and other computationally intensive functions may be performed.
A number of vibration sensors and vibration monitoring devices exist in the prior art. For example, in one prior art device, vibrations are measured at a single point on a machine using a vibration data logger. The data logger is a self-contained unit that includes a vibration sensing element together with an analog-to-digital converter and random access memory (RAM). This device is secured to the machine, and stores a record of the vibrations measured over a period of time. The device may be subsequently detached from the machine in order to download stored vibration data to a personal computer for analysis.
Further examples of other prior art vibration sensors or vibration monitoring devices are described in the following references: U.S. Pat. No. 5,309,149; U.S. Pat. No. 5,847,658; U.S. Pat. No. 6,032,109; U.S. Pat. No. 6,205,872; U.S. Pat. No. 6,295,510; U.S. Pat. No. 6,297,742; and U.S. Pat. No. 6,601,005.